The present invention relates to the aleviation or at least reduction and attenuation of pain that occurs during lithotripsy of patients who have not been anesthesized, the lithotripsy being carried out by means of acoustic shock waves and the inventive method and system is to be usable regardless of whether patient and shock wave source are submerged in a water bath or if the acoustic coupling is carried out through a limited water path without submersion of patient and equipment.
Shock wave lithotripsy will induce considerable pain if the patient is not anesthesized. As Schlieren investigations and pressure measurements have indicated the pain is attributable to a slow (i.e. long duration) pressure wave following immediately the shock wave proper; and due to the relative long lasting effect of this pressure wave (several milliseconds) the pain threshold is exceeded. The shock wave proper has a much shorter duration such as a half a microsecond. This shock wave comminutes a concrement in the body and because of its short duration it is hardly noticeable in terms of pain.
The production of shock waves under utilization of a submerged i.e. under water spark discharge or a corona discharge under water is, however, causally related to the formation of low frequency pressure components, because the expansion of the spark plasma to a relative large gaseous volume, within a water bath amounts to a rapid displacement of the surrounding water. This displacement results in low frequency pressure pulses of waxing and waning flows, and they can unimpededly escape through the opening in the ellipsoidal reflector facing the patient and reach the patient and act in effect like an impact blow.
German printed patent application No. 29 13 251 discloses a structure by means of which the shock wave generator is closed physically through a planar metallic membrane. However, it was found that the pressure wave causes unwanted bulging of the planar metal membrane, and upon recoiling of the membrane skin tissue can actually be clamped. This clamping of tissue can be avoided by utilization of a water bag being disposed between the membrane and the body of the patient, but that does not alleviate or attenuate the pain producing capability of the pressure wave itself.
German printed Pat. No. 21 51 247 (U.S. Pat. No. 3,942,531) discloses as an alternative to close the shock wave generator by means of an elastic membrane which abuts without formation of an air gap the body of the patient and, therefore, follows the contour of the body and the skin and forms certain natural bulges. This membrane avoids the clamping effect noticed above, but still passes unattenuated the shock wave as well as a subsequent pressure wave so that again pain is not reduced.